On 10/16/18 12:33 AM, Joel Fernandes wrote: > On Mon, Oct 15, 2018 at 02:42:09AM -0700, Christoph Hellwig wrote: >> On Fri, Oct 12, 2018 at 06:31:58PM -0700, Joel Fernandes (Google) wrote: >>> Android needs to mremap large regions of memory during memory management >>> related operations. >> >> Just curious: why? > > In Android we have a requirement of moving a large (up to a GB now, but may > grow bigger in future) memory range from one location to another. I think Christoph's "why?" was about the requirement, not why it hurts applications. I admit I'm now also curious :) > This move > operation has to happen when the application threads are paused for this > operation. Therefore, an inefficient move like it is now (for example 250ms > on arm64) will cause response time issues for applications, which is not > acceptable. Huge pages cannot be used in such memory ranges to avoid this > inefficiency as (when the application threads are running) our fault handlers > are designed to process 4KB pages at a time, to keep response times low. So > using huge pages in this context can, again, cause response time issues. > > Also, the mremap syscall waiting for quarter of a second for a large mremap > is quite weird and we ought to improve it where possible.
